The present invention relates generally to a new and improved television audio transmission system and is particularly directed to apparatus and methods for transmitting and receiving an audio signal which is compatible with existing television audio receiving circuits and which comprises a series of components, including stereophonic components, efficiently utilizing the audio bandwidth of a broadcast television channel.
Under present television broadcasting standards, a band of frequencies approximately 80 KHz wide is designated within each 6 MHz television channel for the transmission of the audio component of a television signal. Within this band of frequencies, an RF main audio carrier signal is frequency modulated by an audio baseband signal for producing a monaural audio transmission signal. The transmitted monaural audio signal is received by a television receiver which converts the RF audio carrier signal to a signal having a frequency centered at 4.5 MHz. The converted 4.5 MHz sound carrier is then processed by an FM detector to reproduce the monaural audio signal which was used to frequency modulate the RF audio carrier at the transmitter. In this regard, it has been recognized that the audio bandwidth of a television channel has heretofore been largely underutilized whereby the opportunity to transmit a substantial amount of information in addition to the conventional monaural signal over this frequency band has not been taken advantage of. It is accordingly a basic object of the present invention to provide a system more fully exercising the audio bandwidth of a conventional television channel and which is compatible with present-day television receivers. More particularly, it is an object of the present invention to provide apparatus for transmitting and receiving information over the audio bandwidth of a television channel which information includes stereophonic sound components as well as a number of additional audio information components.
The transmission of stereophonic audio signals has been popular in radio broadcasting for some time, the basic FCC approved system being disclosed in U.S. Pat. No. 3,257,511 to R. Adler et al. In this system, the arithmetic sum of left (L) and right (R) audio source signals (L+R), commonly referred to as the main channel modulation, is used to directly frequency modulate the RF carrier signal. The difference between the left and right stereophonically related signals (L-R) is used to amplitude modulate a 38 KHz subcarrier signal in a suppressed carrier fashion with the resultant double-sideband signal being impressed as frequency modulation on the radiated RF carrier. In addition, a pilot subcarrier signal of 19 KHz is transmitted for synchronization of the FM receiver. The FM receiver extracts the 19 KHz pilot subcarrier, doubles its frequency, and applies the resulting 38 KHz signal to a synchronous detector where the (L-R) difference signal is recovered from the amplitude modulated 38 KHz stereophonic subcarrier. The recovered (L-R) modulation is then suitably matrixed with the (L+R) main channel modulation in order to recover the original left and right stereophonic signals.
The foregoing stereophonic radio broadcasting system often also includes an SCA component which allows broadcasters to provide a subscription background music service. The SCA component comprises a 67 KHz subcarrier frequency modulated by the background channel program, the frequency modulated subcarrier being used to frequency modulate the main RF carrier signal together with the stereophonic modulation.
Various systems and apparatus have been proposed for the transmission of stereophonic sound together with a conventional television picture transmission. These systems normally utilize the radio broadcasting stereophonic transmission techniques discussed above but with, in most cases, different subcarrier frequencies selected for their compatibility with the transmitted video signal. One such prior art system is disclosed in U.S. Pat. No. 4,048,654 to Wegner. This patent discloses a transmission system in which a composite baseband signal identical to that employed in FM stereophonic radio broadcasting is employed to frequency modulate the main sound carrier of a television transmission signal. Thus, the proposed composite baseband signal includes an (L+R) main channel component, an amplitude modulated double-sideband suppressed-carrier 38 KHz subcarrier (L-R) component and a 19 KHz pilot component. In another embodiment, the use of a subcarrier signal having a frequency equal to 5/4 of the horizontal scanning line frequency (f.sub.H) characterizing the transmitted video signal is proposed in lieu of the 38 KHz (L-R) channel subcarrier to reduce interference from the video component of the television signal.
Another system, which was proposed in U.S. Pat. No. 3,099,707 to R. B. Dome, also employed the conventional stereophonic radio broadcasting system but with an (L-R) channel subcarrier equal to 1.5f.sub.H and a pilot signal equal to 2.5f.sub.H. These frequencies were selected to minimize the effect of the video components of the television signal appearing in the recovered sidebands of the (L-R) channel signal.
U.S. Pat. No. 3,046,329 to Reesor discloses yet another similar system in which the composite baseband signal used to frequency modulate the main sound carrier includes only the main channel (L+R) component and the upper sidebands of the (L-R) channel signal amplitude modulated on a subcarrier having a frequency of 2f.sub.H. Other prior art systems for stereophonic television sound transmission have proposed the use of frequency modulated subcarriers for the (L-R) stereo channel typically centered at 2f.sub.H although a center frequency of 1.5f.sub.H has also been proposed.
As previously mentioned, in addition to transmitting stereophonic sound components on the main aural carrier of a transmitted television signal, it is also desirable to transmit additional information thereby more completely exercising the available audio bandwidth within a television channel. For example, the transmission of a second language audio signal would enable a viewer to selectively operate a television receiver for reproducing the audio signals associated with the transmitted stereophonic information, or alternatively, the audio signals associated with the transmitted second language information. Other examples of such additional information include ENG electronic news gathering) signals and telemetry signals, both of which television broadcasters employ for their own private use. ENG signals are employed to provide a direct communications link between a broadcasters and his station's remote camera crews for real-time news reporting while telemetry signals consist of FCC required remote read-outs from unattended transmitter locations to a control location.
One prior art proposal for providing a second language capability in connection with a transmitted television signal is disclosed in previously mentioned U.S. Pat. No. 4,048,654 to Wegner in which the two channels of a stereophonic-like signal are employed. In particular, the (L+R) main channel signal is used to transmit a first language audio signal and the (L-R) stereo channel signal is used to transmit a second language audio signal. U.S. Pat. No. 3,221,098 to Feldman discloses a transmission system allowing for the simultaneous broadcast of a single television program having up to four or more different language soundtracks by forming a composite baseband signal consisting of four or more different subcarrier signals each amplitude modulated with a different language audio signal, the composite baseband signal being used to frequency modulate the main RF audio carrier. Yet another proposed second language system uses a frequency modulated subcarrier baseband signal centered at 2f.sub.H for both stereophonic sound transmission and for second language transmission. A pilot signal, modulated with one of two different frequencies, is used to indicate which service is being broadcast.
The foregoing systems and techniques for transmitting different audio signals in conjunction with a standard television transmission have not been adopted in the U.S. for a number of reasons including, in certain cases, poor performance and, in others, incompatibility with U.S. television transmission standards. The system of the present invention, on the other hand, comprises a audio transmission system which is fully compatible with U.S. television broadcasting standards and is capable of providing high-fidelity stereophonic sound transmissions together with a high quality second language service. The system is further characterized in that ENG and telemetry signals are also conveniently accommodated within the audio bandwidth of a television channel.